The silly thing about the "Nintendoom" movement, from the beginning, is that they were all gleefully ignoring the simple fact that until Mario 3D World came out, Wii U did not, in point of fact, have a true "system seller". It didn't have a "must have" game. It had many GOOD games. But nothing that people were gonna say "Man now I need a Wii U" for. And even then, a lot MORE people will be finally saying "Man now I need a Wii U", when games like MK8, and Smash Bros., and X come along, depending on their tastes. MK8 and SBU are both going to be major system sellers, I know this because I've lost count of how many people IRL and online that I've heard comment to the effect of "I'll get a Wii U, but I'm waiting for MK/SB to come out before I do". And X, well, if Xenoblade was any barometer, X will be one of the best rpgs of the generation, so for rpg fans that's a big deal (as is SMT x Fire Emblem).
But put simply, Wii U didn't HAVE much in the way of a "killer app". Mario 3D World gives people not only a better glimpse at what the system is capable of graphically (while only scratching the surface, of course), but it also did a pretty solid job of integrating the Gamepad functions (as did Pikmin 3, to an point). So if you think 3D World made a difference, then just wait a few months till MK8 comes out.